The present invention relates to implantable medical devices, and more particularly to implantable medical devices that stimulate the neural system (referred to hereafter as a "neural stimulator" or simply as a "stimulator"). Even more particularly, the invention relates to a neural stimulator that measures and collects data resulting from neural stimulation, and transmits the collected data to a processor for analysis, storage, reporting and/or other purposes. The processor that receives the collected data may be at a non-implanted location remote from the stimulator, at an implanted location adjacent the stimulator, or incorporated as additional circuitry within the same implantable housing as the stimulator.
In an implantable medical device, particularly an implantable neural stimulator, there is a need to measure internal voltages, determine electrode impedances, determine output stimulus linearity, sense and measure biological responses to an electrical impulse, as well as to monitor and measure other biological activities that are associated with or occur coincident with the operation of the device.
Disadvantageously, due to the limited power available within an implantable medical device, coupled with the presence of digital and RF noise in or in the near proximity of the device, the design of any monitoring and sensing circuitry within such device must be non-traditional.
For example, in order to measure a biological response to an applied stimulus (i.e., an "evoked response"), there is a need to deal with the presence of the stimulus artifacts which accompany any applied stimulus. Having the capability of sensing and monitoring the evoked response to an applied stimulus provides a very valuable tool for setting the stimulus parameters at an appropriate level for a given patient. However, heretofore there has been little success in sensing the evoked response because it is such a small signal compared to the stimulus artifact.
By way of example, an evoked response within the aural nerve region may only be in the 10 to 500 microvolt (.mu.v) range. The needed amplification for handling such small signals (which amplification must be on the order of about 1000) makes the amplifier recovery from the artifacts too slow to capture the evoked response, which evoked response onset typically occurs about 30 to 40 microseconds (.mu.s) after the stimulus is applied.
In U.S. Pat. No. 5,531,774 there is disclosed a multichannel cochlear stimulation system of the type with which the present invention may be used. As shown in the '774 patent, the system therein disclosed includes both external (non-implanted) and implanted portions. The implanted portion comprises an implantable cochlear stimulator (ICS) integrally attached to a cochlear electrode array. The electrode array includes a multiplicity, e.g., sixteen, spaced-apart electrodes that may be inserted into a human cochlea, any one of which may be activated for application of an electrical stimulus to cochlear tissue. The ICS disclosed in the '774 patent further includes a back telemetry circuit which allows certain measurements, e.g., voltage levels present within the ICS, or other measured parameters, to be sent back to the external portion of the system. The ICS disclosed in the '774 patent is incorporated herein by reference.
In U.S. Pat. No. 5,758,651, there is disclosed a system whereby the sensing electrodes are open circuited for a selected period of time following delivery of a stimulus in order to avoid sensing the artifact. Disadvantageously, the selected period of time during which the electrodes are open circuited varies from patient to patient, and may vary for a given patient depending upon other conditions. Further, switching circuitry is required to perform the open-circuiting function, which switching may introduce switching transients and glitches into the signal.
Thus, it is seen that there is a need for monitoring circuitry within an implantable neural stimulator, e.g., an implantable cochlear stimulator, that is able to accurately sense the evoked response in the presence of large stimulus artifacts, which stimulus artifacts typically vary in terms of amplitude and duration.